Humans will shrink significantly because of man-made climate change if patterns from previous global warming events are followed, researchers have said.
A team of scientists from the University of Michigan said mammals decreased in size "substantially" during two ancient global warming events, known as hyperthermals.
Shrinkage appeared to be a "common evolutionary response" to extreme global warming events, they said.
Study leader Philip Gingerich found that dwarfing occurred in mammals including primates during the Paleocene-Eocene "thermal maximum period" 55 million years ago.
Mammals also shrank in another climate change 53 million years ago.
Jawbones and tooth fossils from early hoofed mammals and primates showed that horses were about the size of small dogs today. Over the following millennia the animals grew back to their pre-warming sizes.
Predictable natural response
"The fact that it happened twice significantly increases our confidence that we're seeing cause and effect - that one interesting response to global warming in the past was a substantial decrease in body size in mammalian species," Gingerich said.
The reason behind the shrinking was not known, although Gingerich said that lower nutritional value of plants in elevated carbon dioxide levels might be a cause.
Team member Will Clyde said the parallels between ancient hyperthermals and modern climate change made studies of fossil records extremely valuable. "Developing a better understanding of the relationship between mammalian body size change and greenhouse gas-induced global warming during the geological past may help us predict ecological changes that may occur in response to current changes in Earth's climate," he said.